One method of calculating blood pressure employing an electronic sphygmomanometer is an oscillometric method for calculating blood pressure by deflating an arm band (cuff) containing a fluid bag that is wrapped around a part of a living body, and taking the change in volume of the fluid bag conveyed through the change in volume of the compressed blood vessels as a change in pressure (pressure pulse wave amplitude) of the fluid bag.
A characteristic of the fluid bag is that the pressure of the fluid bag and the volume of the fluid bag are related as shown in FIG. 24. That is, referring to FIG. 24, in the area shown in portion A where the pressure of the fluid bag is low, the volume of the fluid bag increases sharply relative to the increase in pressure of the fluid bag. Also, as the pressure of the fluid bag increases, the rate of increase of the volume of the fluid bag gradually decreases relative to the increase in pressure of the fluid bag, as shown in portion B. FIG. 25 and FIG. 26 are diagrams respectively representing a change in volume of the fluid bag (B), a change of fluid density in the fluid bag (C), and a change in pressure of the fluid bag (D) following a change in volume of the blood vessels (A) when the fluid density in the fluid bag is low and when the fluid density in the fluid bag is high. Also, FIG. 27 and FIG. 28 are diagrams respectively representing a change in volume of the fluid bag (B) and a change in pressure of the fluid bag (C) following a change in volume of the blood vessels (A) when the discharge speed of fluid from the fluid bag is fast, that is, when the discharge rate per unit time is large, and when the discharge speed of fluid from the fluid bag is slow, that is, when the discharge rate per unit time is small. The following features can be interpreted from FIG. 25 to FIG. 28 in relationship to the detection accuracy of changes in the volume of blood vessels:    (1) Fluid density in the fluid bag is higher, the higher the pressure of the fluid bag;    (2) Because the change of fluid density in the fluid bag following a change in volume of the blood vessels is smaller the larger the volume of the fluid bag, the detection accuracy of changes in the volume of blood vessels is low;    (3) Because the change of fluid density in the fluid bag following a change in volume of the fluid bag is larger the higher the pressure in the fluid bag, the detection accuracy of changes in the volume of blood vessels increases, in the case of the same change in volume of the fluid bag;    (4) Because the size of the change in volume of the fluid bag resulting from a change in volume of the blood vessels changes depending on the discharge rate of fluid in the fluid bag, the detection accuracy of changes in the volume of blood vessels differs, even for the same pressure of the fluid bag; and    (5) Because a change in volume of the fluid bag resulting from a change in volume of the blood vessels is smaller the greater the discharge rate of fluid in the fluid bag, the detection accuracy of changes in the volume of blood vessels decreases.
Thus, with an electronic sphygmomanometer using the oscillometric method, the detection accuracy of changes in the volume of blood vessels is dependent on the fluid density in the fluid bag and the discharge rate of fluid from the fluid bag.
As shown in FIG. 29A to FIG. 29C, with a sphygmomanometer that deflates the fluid bag at a constant speed, the amount of fluid discharged from the fluid bag is controlled with a valve, according to variables such as the pressure of the fluid bag and the circumference of the measurement site (FIG. 29B), in order to deflate the fluid bag at a constant speed (FIG. 29A). Thereby, as shown in FIG. 29C, the pressure pulse wave amplitude relative to a constant change in volume of the blood vessels is larger in the area where the pressure of the fluid bag is higher, and the pressure pulse wave amplitude relative to a constant change in volume of the blood vessels is smaller in the area where the pressure of the fluid bag is lower. Also, because the amount of change in volume of the blood vessels following a change in pressure of the fluid bag differs depending on the circumference of the measurement site, these factors cause errors in blood pressure measurement.
JP 6-245911A (Patent Document 1) discloses technology for adjusting the discharge rate of a valve according to the circumference of the measurement site, or technology for providing a fluid storage unit in communication with the fluid bag, and performs control while holding the volume sum of the fluid bag and the fluid storage unit constant according to the wrapped circumference of the fluid bag around at the measurement site. The deflation speed can thereby be held constant, even if the circumference of the measurement site differs.
Also, JP 5-329113A (Patent Document 2) discloses a method in which volume change characteristics of the fluid bag relative to the pressure of the fluid bag are provided in advance, a signal indicating a change in pressure of the fluid bag is reconverted to a change in volume, and a blood pressure value is measured using the change in volume.
Also, JP 4-250133A (Patent Document 3) discloses a method of closing a valve for discharging fluid in the fluid bag to prevent attenuation of the change in volume of the blood vessels following a change in volume of the fluid bag in a pulse wave arrival interval.
Patent Document 1: JP 6-245911A
Patent Document 2: JP 5-329113A
Patent Document 3: JP 4-250133A